Titanium-containing alloys and associated methods of manufacture

ABSTRACT

Titanium-containing alloys are generally described. The titanium-containing alloys are, according to certain embodiments, nanocrystalline. According to certain embodiments, the titanium-containing alloys have high relative densities. The titanium-containing alloys can be relatively stable, according to certain embodiments. Inventive methods for making titanium-containing alloys are also described herein. The inventive methods for making titanium-containing alloys can involve, according to certain embodiments, sintering nanocrystalline particulates comprising titanium and at least one other metal to form a titanium-containing nanocrystalline alloy.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/331,273, filed Mar. 7, 2019, and entitled “Titanium-Containing Alloysand Associated Methods of Manufacture,” which is a national stage filingunder 35 U.S.C. § 371 of International Patent Application No.PCT/US2017/050372, filed Sep. 7, 2017, and entitled “Titanium-ContainingAlloys and Associated Methods of Manufacture,” which claims priorityunder 35 U.S.C. § 119(e) to U.S. Provisional Application No. 62/384,232,filed Sep. 7, 2016, and entitled “Stable Nano-Duplex Titanium-MagnesiumAlloys,” each of which is incorporated herein by reference in itsentirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Grant No.W911NF-14-1-0539 awarded by the Army Research Office (ARO). TheGovernment has certain rights in the invention.

TECHNICAL FIELD

Titanium-containing alloys and associated methods of manufacture aregenerally described.

BACKGROUND

Nanocrystalline materials can be susceptible to grain growth. In certaininstances, prior sintering techniques for titanium-based alloys havemade it difficult to produce nanocrystalline materials, including bulknanocrystalline materials, that have both small grain sizes and highrelative densities. Improved systems and methods, and associated metalalloys, would be desirable.

SUMMARY

Titanium-containing alloys are generally described. Thetitanium-containing alloys are, according to certain embodiments,nanocrystalline. According to certain embodiments, thetitanium-containing alloys have high relative densities. Thetitanium-containing alloys can be relatively stable, according tocertain embodiments. Inventive methods for making titanium-containingalloys are also described herein. The inventive methods for makingtitanium-containing alloys can involve, according to certainembodiments, sintering nanocrystalline particulates comprising titaniumand at least one other metal to form a titanium-containingnanocrystalline alloy. The subject matter of the present inventioninvolves, in some cases, interrelated products, alternative solutions toa particular problem, and/or a plurality of different uses of one ormore systems and/or articles.

According to one aspect, inventive metal alloys are provided.

Certain embodiments are related to a sintered nanocrystalline metalalloy, comprising Ti a second metal, wherein Ti is the most abundantmetal by atomic percentage in the sintered nanocrystalline metal alloy,and the sintered nanocrystalline metal alloy has a relative density ofat least 80%.

According to some embodiments, a sintered nanocrystalline metal alloycomprises Ti and a second metal, wherein the second metal and Ti exhibita miscibility gap, and the sintered nanocrystalline metal alloy has arelative density of at least 80%.

Some embodiments are related to a bulk nanocrystalline metal alloycomprising Ti and a second metal, wherein Ti is the most abundant metalby atomic percentage in the bulk nanocrystalline metal alloy, and thebulk nanocrystalline metal alloy is substantially stable at atemperature that is greater than or equal to 100° C.

Certain embodiments are related to a bulk nanocrystalline metal alloycomprising Ti and a second metal, wherein Ti is the most abundant metalby atomic percentage in the bulk nanocrystalline metal alloy, and thebulk nanocrystalline metal alloy has an average grain size of less than300 nm.

According to some embodiments, a metal alloy comprises Ti and Mg,wherein the metal alloy has a relative density of greater than or equalto 80%. In some such embodiments, the metal alloy comprises anano-duplex structure comprising or consisting of Ti-rich grains andMg-rich precipitates.

In another aspect, methods of forming metal alloys are provided.

In accordance with some embodiments, a method of forming ananocrystalline metal alloy comprises sintering a plurality ofnanocrystalline particulates to form the nanocrystalline metal alloy,wherein at least some of the nanocrystalline particulates comprise Tiand a second metal, and Ti is the most abundant metal by atomicpercentage in at least some of the nanocrystalline particulates.

According to certain embodiments, a method of forming a nanocrystallinemetal alloy comprises sintering a plurality of nanocrystallineparticulates to form the nanocrystalline metal alloy; wherein at leastsome of the nanocrystalline particulates comprise Ti and a second metal;and sintering the plurality of nanocrystalline particulates involvesheating the nanocrystalline particulates to a first sinteringtemperature that is greater than or equal to 300° C. and less than orequal to 850° C. for a sintering duration greater than or equal to 10minutes and less than or equal to 24 hours.

In some embodiments, a method of forming a nanocrystalline metal alloycomprises sintering a plurality of nanocrystalline particulates to formthe nanocrystalline metal alloy; wherein at least some of thenanocrystalline particulates comprise Ti and a second metal; andsintering the plurality of nanocrystalline particulates involves heatingthe nanocrystalline particulates such that the nanocrystallineparticulates are not at a temperature of greater than or equal to 1200°C. for more than 24 hours.

In accordance with certain embodiments, a method of forming ananocrystalline metal alloy comprises sintering a plurality ofnanocrystalline particulates to form the nanocrystalline metal alloy;wherein at least some of the nanocrystalline particulates comprise Tiand a second metal; Ti is the most abundant metal by atomic percentagein at least some of the nanocrystalline particulates; and the sinteringcomprises heating the nanocrystalline particulates to a first sinteringtemperature lower than a second sintering temperature needed forsintering Ti in the absence of the second metal.

In some embodiments, a method of forming a nanocrystalline metal alloycomprises sintering a plurality of nanocrystalline particulates to formthe nanocrystalline metal alloy; wherein at least some of thenanocrystalline particulates comprise Ti and a second metal; and thesecond metal and Ti exhibit a miscibility gap.

In certain embodiments, a method of forming a nanocrystalline metalalloy comprises sintering a plurality of nanocrystalline particulates toform the nanocrystalline metal alloy, wherein at least some of thenanocrystalline particulates comprise Ti and a second metal; Ti is themost abundant metal by atomic percentage in at least some of thenanocrystalline particulates; and the nanocrystalline metal alloy has arelative density of at least 80%.

In accordance with some embodiments, a method of forming a metal alloycomprises sintering powder comprising Ti and Mg to produce the metalalloy, wherein the metal alloy has a relative density of greater than orequal to 80%. In some such embodiments, the method further comprisesmilling powders of elemental Ti and Mg. For example, powders ofelemental Ti and Mg can be mixed and milled (e.g., to achievesupersaturation and a decrease of the grain size to the nanometerscale). In some such embodiments, the powders can be compressed prior tosintering. According to some such embodiments, a nano-duplex structureconsisting of Ti-rich grains and Mg-rich precipitates is developed.

Other advantages and novel features of the present invention will becomeapparent from the following detailed description of various non-limitingembodiments of the invention when considered in conjunction with theaccompanying figures. In cases where the present specification and adocument incorporated by reference include conflicting and/orinconsistent disclosure, the present specification shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. In the figures,each identical or nearly identical component illustrated is typicallyrepresented by a single numeral. For purposes of clarity, not everycomponent is labeled in every figure, nor is every component of eachembodiment of the invention shown where illustration is not necessary toallow those of ordinary skill in the art to understand the invention. Inthe figures:

FIGS. 1A-1C are schematic diagrams showing a sintering process,according to certain embodiments;

FIG. 2 is a plot of the enthalpy of segregation, ΔH^(seg) (kJ/mol) vs.the enthalpy of mixing, AB′ (kJ/mol) of various metals with titanium,according to some embodiments;

FIG. 3 shows a series of x-ray diffraction (XRD) spectra fornanocrystalline powder samples, according to certain embodiments;

FIG. 4 is a plot of grain sizes and lattice parameters ofnanocrystalline powders that contained titanium and 10 at. % Mg, 20 at.% Mg, and 30 at. % Mg, according to some embodiments;

FIG. 5 is, in accordance with some embodiments, a series of TEM imagesand corresponding electron diffraction patterns for nanocrystallinepowders that contained titanium and 10 at. % Mg, 20 at. % Mg, and 30 at.% Mg;

FIG. 6 is, according to some embodiments, an electron diffractionpattern from a TEM of Ti-20 at. % Mg;

FIGS. 7A-7B are a set of photographs of samples that were powderscontaining different atomic percentages of titanium and magnesium thathad been cold pressed and covered with a tantalum (Ta) foil (FIG. 7A) ora copper (Cu) tube (FIG. 7B), according to certain embodiments;

FIG. 8 is, according to some embodiments, a plot of relative density asa function of applied load;

FIG. 9 is, in accordance with some embodiments, a plot of the change inrelative density as a function of sintering temperature;

FIGS. 10A-10C show scanning transmission electron microscopy-energydispersive x-ray spectroscopy (STEM-EDS) images for Ti-20 at % Mg aftersintering at 500° C. for 8 h, according to certain embodiments;

FIG. 11 is an XRD plot before (dotted) and after (solid lines)sintering, according to certain embodiments;

FIG. 12 is an STEM image of a metal alloy after sintering, according tosome embodiments; and

FIG. 13 is an STEM image of a metal alloy after sintering, according tocertain embodiments.

DETAILED DESCRIPTION

Nanocrystalline metals have certain advantages over theirmicrocrystalline counterparts due to the large volume fraction of grainboundaries. As one example, nanocrystalline alloys generally haveremarkably higher tensile strength. However, nanocrystalline metals haveprimarily been processed as thin films, as retaining nanoscale grains inprocessing a bulk material is much more difficult.

This disclosure is generally directed to metal alloys comprisingtitanium. The metal alloys comprising the titanium are, according tocertain embodiments, nanocrystalline metal alloys. Certain of the metalalloys described herein can have high relative densities whilemaintaining their nanocrystalline character. In addition, according tocertain embodiments, the metal alloys can be bulk metal alloys. Certainof the metal alloys described herein are stable against grain growth.

Inventive methods for making titanium-containing alloys are alsodescribed herein. For example, certain embodiments are directed tosintering methods in which the sintering is achieved at relatively lowtemperatures and/or over a relatively short period of time. According tosome embodiments, and as described in more detail below, the sinteringcan be performed such that undesired grain growth is limited oreliminated (e.g., via the selection of materials and/or sinteringconditions). Certain embodiments are directed to the recognition thatone can sinter titanium-containing materials over relatively short timesand/or at relatively low temperatures while maintainingnanocrystallinity.

Certain of the embodiments described herein can provide advantagesrelative to prior articles, systems, and methods. For example, accordingto certain (although not necessarily all) embodiments, thetitanium-containing metal alloys can have high strength, high hardness,and/or high resistance to grain growth. According to some (although notnecessarily all) embodiments, the methods for forming metal alloysdescribed herein can make use of relatively small amounts of energy, forexample, due to the relatively short sintering times and/or therelatively low sintering temperatures that are employed.

As noted above, certain embodiments are related to inventive metalalloys. The metal alloys comprise, according to certain embodiments,titanium and at least one other metal.

According to certain embodiments, the metal alloy comprises titanium(Ti). The metal alloy can contain, according some embodiments, arelatively large amount of titanium. For example, in some embodiments,Ti is the most abundant metal by atomic percentage in the metal alloy.(Atomic percentages are abbreviated herein as “at. %” or “at %”.)According to certain embodiments, Ti is present in the metal alloy in anamount of at least 50 at. %, at least 55 at. %, at least 60 at. %, atleast 70 at. %, at least 80 at. %, at least 90 at. %, or at least 95 at.%. In some embodiments, Ti is present in the metal alloy in an amount ofup to 96 at. %, up to 97 at. %, up to 98 at. %, or more. Combinations ofthese ranges are also possible. Other values are also possible.

The metal alloys described herein can comprise a second metal. Thephrase “second metal” is used herein to describe any metal element thatis not Ti. The term “element” is used herein to refer to an element asfound on the Periodic Table. “Metal elements” are those found in Groups1-12 of the Periodic Table except hydrogen (H); Al, Ga, In, Tl, and Nhin Group 13 of the Periodic Table; Sn, Pb, and Fl in Group 14 of thePeriodic Table; Bi and Mc in Group 15 of the Periodic Table; Po and Lvin Group 16 of the Periodic Table; the lanthanides; and the actinides.In some embodiments, the second metal is a refractory metal element(e.g., Nb, Ta, Mo, W, and/or Re). In some embodiments, the second metalis a transition metal (i.e., any of those in Groups 3-12 of the PeriodicTable). In some embodiments, the second metal is a lanthanide (anelement with the atomic number 57-71, inclusive). In some embodiments,the second metal is a rare earth element, e.g. Scandium (Sc), Yttrium(Y), or a lanthanide. In some embodiments, the second metal is anactinide (an element with the atomic number 89-103, inclusive).According to certain embodiments, the second metal is selected from thegroup consisting of lithium (Li), sodium (Na), potassium (K), rubidium(Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg),calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), scandium (Sc),yttrium (Y), Lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium(Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd),terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), lutetium (Lu), Actinium (Ac), thorium (Th), protactinium(Pa), uranium (U), neptunium (Np), plutonium (Pu), americium (Am),curium (Cm), berkelium (Bk), californium (CO, einsteinium (Es), fermium(Fm), mendelevium (Md), nobelium (No), and lawrencium (Lr), zirconium(Zr), hafnium (Hf), rutherfordium (Rf), vanadium (V), niobium (Nb),tantalum (Ta), dubnium (Db), chromium (Cr), molybdenum (Mo), tungsten(W), seaborgium (Sg), manganese (Mn), technetium (Tc), rhenium (Re),bohrium (Bh), iron (Fe), ruthenium (Ru), osmium (Os), hassium (Hs),cobalt (Co), rhodium (Rh), iridium (Ir), meitnerium (Mt), nickel (Ni),palladium (Pd), platinum (Pt), darmstadtium (Ds), copper (Cu), silver(Ag), gold (Au), roentgenium (Rg), zinc (Zn), cadmium (Cd), mercury(Hg), copernicium (Cn), aluminum (Al), gallium (Ga), indium (In),thallium (Tl), nihonium (Nh), tin (Sn), lead (Pb), flerovium (Fl),bismuth (Bi), moscovium (Mc), polonium (Po), and livermorium (Lv). Themetal alloy can comprise, in some embodiments, combinations of two ormore of these.

According to certain embodiments, the second metal comprises an alkalineearth metal. The phrase “alkaline earth metal” is used herein todescribe the elements in Group 2 of the Periodic Table (i.e., Be, Mg,Ca, Sr, Ba, and Ra). In certain embodiments, the second metal isselected from the group consisting of Mg, La, Y, Th, Sc, Cr, Ag, Fe, Mn,Cu, and Li. In some embodiments, the second metal is Mg.

According to certain embodiments, the second metal and Ti exhibit amiscibility gap. Two elements are said to exhibit a “miscibility gap”when the phase diagram of those two elements includes a region in whichthe mixture of the two elements exists as two or more phases. In someembodiments in which the second metal and Ti exhibit a miscibility gap,the second metal and Ti can be present in the metal alloy among at leasttwo phases.

In some embodiments, Ti is at least partially soluble in the secondmetal. For example, in some embodiments, Ti and the second metal are ina solid solution.

The second metal may be present in the metal alloy in a variety ofsuitable percentages. According to certain embodiments, the second metalis present in the metal alloy in an amount of less than or equal to 40at. %, less than or equal to 35 at. %, less than or equal to 32 at. %,less than or equal to 30 at. %, less than or equal to 25 at. %, lessthan or equal to 22 at. %, less than or equal to 20 at. %, less than orequal to 15 at. %, or less than or equal to 12 at. %. In someembodiments, the second metal is present in the metal alloy in an amountof at least 1 at. %, at least 2 at. %, at least 3 at. %, at least 4 at.%, at least 5 at. %, at least 6 at. %, at least 7 at. %, at least 8 at.%, at least 9 at. %, at least 10 at. %, or more. Combinations of theseranges are also possible. For example, in some embodiments, the secondmetal is present in the metal alloy in an amount of from 1 at. % to 40at. % of the metal alloy. In some embodiments, the second metal ispresent in the metal alloy in an amount of from 8 at. % to 32 at. % ofthe metal alloy. Other values are also possible.

In some embodiments, the second metal may be an activator element,relative to Ti. Activator elements are those elements that increase therate of sintering of a material, relative to sintering rates that areobserved in the absence of the activator element but under otherwiseidentical conditions. Activator elements are described in more detailbelow.

In certain embodiments, the second metal may be a stabilizer element,relative to Ti. Stabilizer elements are those elements that reduce therate of grain growth of a material, relative to grain growth rates thatare observed in the absence of the stabilizer element but underotherwise identical conditions. Stabilizer elements are described inmore detail below. In some embodiments, the second metal may be both astabilizer element and an activator element.

According to certain embodiments, the second metal (e.g., for forming analloy with Ti) can be selected based on one or more of the followingconditions:

1. thermodynamic stabilization of the nanocrystalline grain size;

2. phase separation region, which is extended above the sinteringtemperature;

3. second (e.g., solute) element with lower melting temperature; and/or

4. solubility of the Ti in the precipitated second phase.

According to some embodiments, the second metal (e.g., Mg) forms anano-duplex structure with the Ti. For example, in some embodiments, themetal alloy comprises a nano-duplex structure consisting of Ti-richgrains and Mg-rich precipitates. In some embodiments, nanocrystallinestructure with grain sizes of around 110 nm can be maintained even after8 hours at 500° C. (which is 84% of the melting temperature for Mg and30% for Ti). According to some embodiments, high relative densities canbe achieved for Ti-20 at. % Mg and Ti-30 at. % Mg.

In some embodiments, the metal alloy comprises only Ti and the secondmetal (i.e., Ti and the second metal without additional metals or otherelements). In other embodiments, the metal alloy comprises Ti, thesecond metal, and a third element. For example, in some embodiments, themetal alloy comprises a third metal (in addition to Ti and the secondmetal). The phrase “third metal” is used herein to describe a metal thatis not Ti and that is not the second metal.

The third metal may be present in the metal alloy in a variety ofsuitable percentages. According to certain embodiments, the third metalis present in the metal alloy in an amount of less than or equal to 40at. %, less than or equal to 35 at. %, less than or equal to 32 at. %,less than or equal to 30 at. %, less than or equal to 25 at. %, lessthan or equal to 22 at. %, less than or equal to 20 at. %, less than orequal to 15 at. %, or less than or equal to 12 at. %. In someembodiments, the third metal is present in the metal alloy in an amountof at least 1 at. %, at least 2 at. %, at least 3 at. %, at least 4 at.%, at least 5 at. %, at least 6 at. %, at least 7 at. %, at least 8 at.%, at least 9 at. %, at least 10 at. %, or more. Combinations of theseranges are also possible. Other values are also possible.

According to certain embodiments, the third metal may be a stabilizerelement, an activator element, or both a stabilizer element and anactivator element.

In some embodiments, the metal alloy comprises Ti and at least one ofMg, La, Y, Th, Sc, Cr, Ag, Fe, Mn, Cu, and Li. In some embodiments, themetal alloy comprises Ti, Mg, and at least one of La, Y, Th, Sc, Cr, Ag,Fe, Mn, Cu, and Li.

According to certain embodiments, the total amount of all metal elementsin the metal alloy that are not Ti (e.g., the second metal, the optionalthird metal, and any additional optional metals) makes up less than 50at. %, less than or equal to 40 at. %, less than or equal to 35 at. %,less than or equal to 32 at. %, less than or equal to 30 at. %, lessthan or equal to 25 at. %, less than or equal to 22 at. %, less than orequal to 20 at. %, less than or equal to 15 at. %, or less than or equalto 12 at. % of the metal alloy. In some embodiments, the total amount ofall metal elements in the metal alloy that are not Ti (e.g., the secondmetal, the optional third metal, and any additional optional metals)makes up at least 1 at. %, at least 2 at. %, at least 3 at. %, at least4 at. %, at least 5 at. %, at least 6 at. %, at least 7 at. %, at least8 at. %, at least 9 at. %, at least 10 at. %, or more. Combinations ofthese ranges are also possible. Other values are also possible.

According to certain embodiments, the metal alloys are nanocrystallinemetal alloys. Nanocrystalline materials generally refer to materialsthat comprise at least some grains with a grain size smaller than orequal to 1000 nm. In some embodiments, the nanocrystalline materialcomprises grains with a grain size smaller than or equal to 900 nm,smaller than or equal to 800 nm, smaller than or equal to 700 nm,smaller than or equal to 600 nm, smaller than or equal to 500 nm,smaller than or equal to 400 nm, smaller than or equal to 300 nm,smaller than or equal to 200 nm, smaller than or equal to 100 nm,smaller than or equal to 50 nm, or smaller than or equal to 20 nm.Accordingly, in the case of metal alloys, nanocrystalline metal alloysare metal alloys that comprise grains with a grain size smaller than orequal to 1000 nm. In some embodiments, the nanocrystalline metal alloycomprises grains with a grain size smaller than or equal to 900 nm,smaller than or equal to 800 nm, smaller than or equal to 700 nm,smaller than or equal to 600 nm, smaller than or equal to 500 nm,smaller than or equal to 400 nm, smaller than or equal to 300 nm,smaller than or equal to 200 nm, smaller than or equal to 150 nm,smaller than or equal to 125 nm, smaller than or equal to 100 nm,smaller than or equal to 50 nm, or smaller than or equal to 20 nm. Othervalues are also possible.

The “grain size” of a grain generally refers to the largest dimension ofthe grain. The largest dimension may be a diameter, a length, a width,or a height of a grain, depending on the geometry thereof. According tocertain embodiments, the grains may be spherical, cubic, conical,cylindrical, needle-like, or any other suitable geometry.

According to certain embodiments, a relatively large percentage of thevolume of the metal alloy is made up of small grains. For example, insome embodiments, at least 50%, at least 75%, at least 90%, at least95%, at least 99%, or substantially all of the volume of the metal alloyis made up of grains having grain sizes of smaller than or equal to 1000nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm,smaller than or equal to 700 nm, smaller than or equal to 600 nm,smaller than or equal to 500 nm smaller than or equal to 400 nm, smallerthan or equal to 300 nm, smaller than or equal to 200 nm, smaller thanor equal to 150 nm, smaller than or equal to 125 nm, smaller than orequal to 100 nm, smaller than or equal to 50 nm, or smaller than orequal to 20 nm. Other values are also possible.

According to certain embodiments, the metal alloy may have a relativelysmall average grain size. The “average grain size” of a material (e.g.,a metal alloy) refers to the number average of the grain sizes of thegrains in the material. According to certain embodiments, the metalalloy (e.g., a bulk and/or nanocrystalline metal alloy) has an averagegrain size of smaller than or equal to 1000 nm, smaller than or equal to900 nm, smaller than or equal to 800 nm, smaller than or equal to 700nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm,smaller than or equal to 400 nm, smaller than or equal to 300 nm,smaller than or equal to 200 nm, smaller than or equal to 150 nm,smaller than or equal to 125 nm, smaller than or equal to 100 nm,smaller than or equal to 50 nm, or smaller than or equal to 20 nm. Incertain embodiments, the metal alloy has an average grain size of aslittle as 25 nm, as little as 10 nm, as little as 1 nm, or smaller.Combinations of these ranges are also possible. Other values are alsopossible.

According to certain embodiments, at least one cross-section of themetal alloy that intersects the geometric center of the metal alloy hasa small volume-average cross-sectional grain size. The “volume-averagecross-sectional grain size” of a given cross-section of a metal alloy isdetermined by obtaining the cross-section of the object, tracing theperimeter of each grain in an image of the cross-section of the object(which may be a magnified image, such as an image obtained from atransmission electron microscope), and calculating thecircular-equivalent diameter, D_(i), of each traced grain cross-section.The “circular-equivalent diameter” of a grain cross-section correspondsto the diameter of a circle having an area (A, as determined by A=πr²)equal to the cross-sectional area of the grain in the cross-section ofthe object. The volume-average cross-sectional grain size (G_(CS,avg))is calculated as:

$G_{{CS},{avg}} = \left( \frac{\sum_{i = 1}^{i = n}D_{i}^{3}}{n} \right)^{1/3}$

where n is the number of grains in the cross-section and D_(i) is thecircular-equivalent diameter of grain i.

According to certain embodiments, at least one cross-section of themetal alloy that intersects the geometric center of the metal alloy hasa volume-average cross-sectional grain size of smaller than or equal to1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm,smaller than or equal to 500 nm, smaller than or equal to 400 nm,smaller than or equal to 300 nm, smaller than or equal to 200 nm,smaller than or equal to 150 nm, smaller than or equal to 125 nm,smaller than or equal to 100 nm, smaller than or equal to 50 nm, orsmaller than or equal to 20 nm. In certain embodiments, at least onecross-section of the metal alloy that intersects the geometric center ofthe metal alloy has a volume-average cross-sectional grain size of assmall as 25 nm, as small as 10 nm, as small as 1 nm, or smaller.Combinations of these ranges are also possible. Other values are alsopossible.

According to certain embodiments, at least one cross-section of themetal alloy (that, optionally, intersects the geometric center of themetal alloy) has a volume-average cross-sectional grain size of smallerthan or equal to 1000 nm, smaller than or equal to 900 nm, smaller thanor equal to 800 nm, smaller than or equal to 700 nm, smaller than orequal to 600 nm, smaller than or equal to 500 nm, smaller than or equalto 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm,smaller than or equal to 100 nm, smaller than or equal to 50 nm, orsmaller than or equal to 20 nm (and/or as small as 25 nm, as small as 10nm, as small as 1 nm, or smaller); and at least a second cross-sectionof the metal alloy that is orthogonal to the first cross section (that,optionally, intersects the geometric center of the metal alloy) has avolume-average cross-sectional grain size of smaller than or equal to1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm,smaller than or equal to 500 nm smaller than or equal to 400 nm, smallerthan or equal to 300 nm, smaller than or equal to 200 nm, smaller thanor equal to 150 nm, smaller than or equal to 125 nm, smaller than orequal to 100 nm, smaller than or equal to 50 nm, or smaller than orequal to 20 nm (and/or as small as 25 nm, as small as 10 nm, as small as1 nm, or smaller). Other values are also possible.

According to certain embodiments, at least one cross-section of themetal alloy (that, optionally, intersects the geometric center of themetal alloy) has a volume-average cross-sectional grain size of smallerthan or equal to 1000 nm, smaller than or equal to 900 nm, smaller thanor equal to 800 nm, smaller than or equal to 700 nm, smaller than orequal to 600 nm, smaller than or equal to 500 nm, smaller than or equalto 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm,smaller than or equal to 100 nm, smaller than or equal to 50 nm, orsmaller than or equal to 20 nm (and/or as small as 25 nm, as small as 10nm, as small as 1 nm, or smaller); at least a second cross-section ofthe metal alloy that is orthogonal to the first cross section (that,optionally, also intersects the geometric center of the metal alloy, orotherwise) has a volume-average cross-sectional grain size of smallerthan or equal to 1000 nm, smaller than or equal to 900 nm, smaller thanor equal to 800 nm, smaller than or equal to 700 nm, smaller than orequal to 600 nm, smaller than or equal to 500 nm, smaller than or equalto 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm,smaller than or equal to 100 nm, smaller than or equal to 50 nm, orsmaller than or equal to 20 nm (and/or as small as 25 nm, as small as 10nm, as small as 1 nm, or smaller); and at least a third cross-section ofthe metal alloy that is orthogonal to the first cross-section and thatis orthogonal to the second cross-section (that, optionally, alsointersects the geometric center of the metal alloy) has a volume-averagecross-sectional grain size of smaller than or equal to 1000 nm, smallerthan or equal to 900 nm, smaller than or equal to 800 nm, smaller thanor equal to 700 nm, smaller than or equal to 600 nm, smaller than orequal to 500 nm, smaller than or equal to 400 nm, smaller than or equalto 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm,smaller than or equal to 50 nm, or smaller than or equal to 20 nm(and/or as small as 25 nm, as small as 10 nm, as small as 1 nm, orsmaller).

In some embodiments, the metal alloy comprises grains that arerelatively equiaxed. In certain embodiments, at least a portion of thegrains within the metal alloy have aspect ratios of less than or equalto 2, less than or equal to 1.8, less than or equal to 1.6, less than orequal to 1.4, less than or equal to 1.3, less than or equal to 1.2, orless than or equal to 1.1 (and, in some embodiments, down to 1). Theaspect ratio of a grain is calculated as the maximum cross-sectionaldimension of the grain which intersects the geometric center of thegrain, divided by the largest dimension of the grain that is orthogonalto the maximum cross-sectional dimension of the grain. The aspect ratioof a grain is expressed as a single number, with 1 corresponding to anequiaxed grain. In some embodiments, the number average of the aspectratios of the grains in the metal alloy is less than or equal to 2, lessthan or equal to 1.8, less than or equal to 1.6, less than or equal to1.4, less than or equal to 1.3, less than or equal to 1.2, or less thanor equal to 1.1 (and, in some embodiments, down to 1).

Without wishing to be bound by any particular theory, it is believedthat relatively equiaxed grains may be present when the metal alloy isproduced in the absence (or substantial absence) of applied pressure(e.g., via a pressureless or substantially pressureless sinteringprocess).

In certain embodiments, the metal alloy comprises a relatively lowcross-sectional average grain aspect ratio. In some embodiments, thecross-sectional average grain aspect ratio in the metal alloy is lessthan or equal to 2, less than or equal to 1.8, less than or equal to1.6, less than or equal to 1.4, less than or equal to 1.3, less than orequal to 1.2, or less than or equal to 1.1 (and, in some embodiments,down to 1). The “cross-sectional average grain aspect ratio” of a metalalloy is said to fall within a particular range if at least onecross-section of the metal alloy that intersects the geometric center ofthe metal alloy is made up of grain cross-sections with an averageaspect ratio falling within that range. For example, the cross-sectionalaverage grain aspect ratio of a metal alloy would be less than 2 if themetal alloy includes at least one cross-section that intersects thegeometric center of the metal alloy and in which the cross-section ismade up of grain cross-sections with an average aspect ratio of lessthan 2. To determine the average aspect ratio of the graincross-sections from which the cross-section of the metal alloy is madeup (also referred to herein as the “average aspect ratio of graincross-sections”), one obtains the cross-section of the metal alloy,traces the perimeter of each grain in an image of the cross-section ofthe metal alloy (which may be a magnified image, such as an imageobtained from a transmission electron microscope), and calculates theaspect ratio of each traced grain cross-section. The aspect ratio of agrain cross-section is calculated as the maximum cross-sectionaldimension of the grain cross-section (which intersects the geometriccenter of the grain cross-section), divided by the largest dimension ofthe grain cross-section that is orthogonal to the maximumcross-sectional dimension of the grain cross-section. The aspect ratioof a grain cross-section is expressed as a single number, with 1corresponding to an equiaxed grain cross-section. The average aspectratio of the grain cross-sections from which the cross-section of themetal alloy is made up (AR_(avg)) is calculated as a number average:

${AR_{avg}} = \frac{\sum_{i = 1}^{i = n}{AR_{i}}}{n}$

where n is the number of grains in the cross-section and AR_(i) is theaspect ratio of the cross-section of grain i.

According to certain embodiments, a metal alloy having a cross-sectionalaverage grain aspect ratio falling within a particular range (e.g., anyof the ranges described elsewhere herein) has a first cross-sectionintersecting the geometric center of the metal alloy and having anaverage aspect ratio of grain cross-sections falling within that range,and at least a second cross-section—orthogonal to the firstcross-section—intersecting the geometric center of the metal alloy andhaving an average aspect ratio of grain cross-sections falling withinthat range. For example, according to certain embodiments, a metal alloyhaving a cross-sectional average grain aspect ratio of less than 2includes a cross-section that intersects the geometric center of themetal alloy having an average aspect ratio of grain cross-sections ofless than 2 and at least a second cross-section—orthogonal to the firstcross-section—intersecting the geometric center of the metal alloy andhaving an average aspect ratio of grain cross-sections of less than 2.

According to certain embodiments, a metal alloy having a cross-sectionalaverage grain aspect ratio falling within a particular range (e.g., anyof the ranges described elsewhere herein) has a first cross-sectionintersecting the geometric center of the metal alloy and having anaverage aspect ratio of grain cross-sections falling within that range;a second cross-section—orthogonal to the firstcross-section—intersecting the geometric center of the metal alloy andhaving an average aspect ratio of grain cross-sections falling withinthat range; and at least a third cross-section—orthogonal to the firstcross-section and the second cross-section—intersecting the geometriccenter of the metal alloy and having an average aspect ratio of graincross-sections falling within that range. For example, according tocertain embodiments, a metal alloy having a cross-sectional averagegrain aspect ratio of less than 2 includes a first cross-section thatintersects the geometric center of the metal alloy having an averageaspect ratio of grain cross-sections of less than 2, a secondcross-section—orthogonal to the first cross-section—intersecting thegeometric center of the metal alloy and having an average aspect ratioof grain cross-sections of less than 2, and at least a thirdcross-section—orthogonal to the first cross-section and the secondcross-section—intersecting the geometric center of the metal alloy andhaving an average aspect ratio of grain cross-sections of less than 2.

According to certain embodiments, the grains within the metal alloy canbe both relatively small and relatively equiaxed. For example, accordingto certain embodiments, at least one cross-section (and, in someembodiments, at least a second cross-section that is orthogonal to thefirst cross-section and/or at least a third cross-section that isorthogonal to the first and second cross-sections) can have avolume-average cross-sectional grain size and an average aspect ratio ofgrain cross-sections falling within any of the ranges outlined above orelsewhere herein.

The metal alloy can, according to certain embodiments, be a bulk metalalloy (e.g., a bulk nanocrystalline metal alloy). A “bulk metal alloy”is a metal alloy that is not in the form of a thin film. In certainembodiments, the bulk metal alloy has a smallest dimension of at least 1micron. In some embodiments, the bulk metal alloy has a smallestdimension of at least 10 microns, at least 25 microns, at least 50microns, at least 100 microns, at least 500 microns, at least 1millimeter, at least 1 centimeter, at least 10 centimeters, at least 100centimeters, or at least 1 meter. Other values are also possible.According to certain embodiments, the metal alloy is not in the form ofa coating.

In certain embodiments, the metal alloy occupies a volume of at least 1mm³, at least 5 mm³, at least 10 mm³, at least 0.1 cm³, at least 0.5cm³, at least 1 cm³, at least 10 cm³, at least 100 cm³, or at least 1m³. Other values are also possible.

According to certain embodiments, the metal alloy comprises multiplephases. For example, in some embodiments, the metal alloy is adual-phase metal alloy.

In some embodiments, the metal alloy has a high relative density. Theterm “relative density” refers to the ratio of the experimentallymeasured density of the metal alloy and the maximum theoretical densityof the metal alloy. The “relative density” (ρ_(rel)) is expressed as apercentage, and is calculated as:

$\rho_{rel} = {{\frac{\rho_{measured}}{\rho_{maximum}} \times 100}\%}$

wherein ρ_(measured) is the experimentally measured density of the metalalloy and ρ_(maximum) is the maximum theoretical density of an alloyhaving the same composition as the metal alloy.

In some embodiments, the metal alloy (e.g., a sintered metal alloy, ananocrystalline metal alloy, and/or a bulk metal alloy) has a relativedensity of at least 80%, at least 85%, at least 90%, at least 92%, atleast 95%, at least 96%, at least 97%, at least 98%, or at least 99%(and/or, in certain embodiments, up to 99.8%, up to 99.9%, or more). Insome embodiments, the nanocrystalline alloy has a relative density of100%. Other values are also possible.

According to certain embodiments, the metal alloy is fully dense. Asutilized herein, the term “fully dense” (or “full density”) refers to amaterial with a relative density of at least 98%. According to certainembodiments, the relative density of the metal alloy may impact othermaterial properties of the metal alloy. Thus, by controlling therelative density of the metal alloy, other material properties of themetal alloy may be controlled.

According to certain embodiments, metal alloys described herein can bestable at relatively high temperatures. A metal alloy is said to be“substantially stable” at a particular temperature when the metal alloyincludes at least one cross-section intersecting the geometric center ofthe alloy in which the volume-average cross-sectional grain size(described above) of the cross-section does not increase by more than20% (relative to the original volume-average cross-sectional grain size)when the metal alloy is heated to that temperature for 24 hours in anargon atmosphere. One of ordinary skill in the art would be capable ofdetermining whether a metal alloy is stable at a particular temperatureby taking a cross-section of the article, determining the volume-averagecross-sectional grain size of the cross-section at 25° C., heating thecross-section to the particular temperature for 24 hours in an argonatmosphere, allowing the cross-section to cool back to 25° C., anddetermining—post-heating—the volume-average cross-sectional grain sizeof the cross-section. The metal alloy would be said to be stable if thevolume-average cross-sectional grain size of the cross-section after theheating step is less than 120% of the volume-average cross-sectionalgrain size of the cross-section prior to the heating step. According tocertain embodiments, a metal alloy that is stable at a particulartemperature includes at least one cross-section intersecting thegeometric center of the metal alloy in which the volume-averagecross-sectional grain size of the cross-section does not increase bymore than 15%, more than 10%, more than 5%, or more than 2% (relative tothe original volume-average grain size) when the object is heated tothat temperature for 24 hours in an argon atmosphere.

In some embodiments, the metal alloy is substantially stable at at leastone temperature that is greater than or equal to 100 degrees Celsius (°C.). In certain embodiments, the metal alloy is substantially stable atat least one temperature that is greater than or equal to 200° C.,greater than or equal to 300° C., greater than or equal to 400° C.,greater than or equal to 500° C., greater than or equal to 600° C.,greater than or equal to 700° C., greater than or equal to 800° C.,greater than or equal to 900° C., greater than or equal to 1000° C.,greater than or equal to 1100° C., greater than or equal to 1200° C.,greater than or equal to 1300° C., or greater than or equal to 1400° C.Other ranges are also possible.

Certain of the metal alloys described herein are sintered metal alloys.Exemplary sintering methods that may be used to produce metal alloysaccording to the present disclosure are described in more detail below.

Also described herein are inventive methods of forming metal alloys(e.g., sintered metal alloys, bulk metal alloys, and/or nanocrystallinemetal alloys). Certain of the inventive methods described herein can beused to form the inventive metal alloys described above and elsewhereherein. For example, certain of the methods described herein can be usedto form nanocrystalline metal alloys, for example, including any of thegrain sizes and/or grain size distributions described above or elsewhereherein. Certain of the methods described herein can be used to formmetal alloys having high relative densities, including any of therelative densities described above or elsewhere herein. Certain of themethods described herein can be used to form bulk nanocrystalline metalalloys, for example, having any of the sizes described above orelsewhere herein. Certain of the methods described herein can be used toform metal alloys that are stable, for example, having any of thestabilities (e.g., against grain growth) described above or elsewhereherein.

In some embodiments, a metal alloy is formed by sintering a plurality ofparticulates. The shape of the particulates may be, for example,spherical, cubical, conical, cylindrical, needle-like, irregular, or anyother suitable geometry. In some embodiments, at least some (e.g., atleast 50%, at least 75%, at least 90%, or at least 95%) of theparticulates are single crystals. In certain embodiments, at least some(e.g., at least 50%, at least 75%, at least 90%, or at least 95%) of theparticulates are polycrystalline.

The particulates that are sintered can be, according to certainembodiments, nanocrystalline particulates. The nanocrystallineparticulates can comprise, according to certain embodiments, grains witha grain size smaller than or equal to 1000 nm, smaller than or equal to900 nm, smaller than or equal to 800 nm, smaller than or equal to 700nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm,smaller than or equal to 400 nm, smaller than or equal to 300 nm,smaller than or equal to 200 nm, smaller than or equal to 150 nm,smaller than or equal to 125 nm, smaller than or equal to 100 nm,smaller than or equal to 50 nm, smaller than or equal to 40 nm, smallerthan or equal to 30 nm, or smaller than or equal to 20 nm. According tocertain embodiments, at least some of the nanocrystalline particulateshave a grain size of smaller than or equal to 50 nm. In someembodiments, at least some of the nanocrystalline particulates have agrain size of greater than or equal to 5 nm and smaller than or equal to25 nm. In some embodiments, at least some of the nanocrystallineparticulates have a grain size of greater than or equal to 10 nm andsmaller than or equal to 20 nm.

According to certain embodiments, at least some of the nanocrystallineparticulates comprise Ti and/or a second metal. In some embodiments, oneportion of the nanocrystalline particulates is made up of Ti whileanother portion of the nanocrystalline particulates are made up of thesecond metal. In certain embodiments, at least some of thenanocrystalline particulates comprise both Ti and the second metal.

In some embodiments, Ti is the most abundant metal by atomic percentagein at least some of the nanocrystalline particulates. In someembodiments, at least some of the particulates contain Ti in an amountof at least 50 at. %, at least 55 at. %, at least 60 at. %, at least 70at. %, at least 80 at. %, at least 90 at. %, or at least 95 at. %. Insome embodiments, at least some of the particulates contain Ti in anamount of up to 96 at. %, up to 97 at. %, up to 98 at. %, or more.Combinations of these ranges are also possible. Other values are alsopossible.

In some embodiments, Ti is the most abundant metal by atomic percentagein the particulate material. According to certain embodiments, the totalamount of Ti present in the particulate material is at least 50 at. %,at least 55 at. %, at least 60 at. %, at least 70 at. %, at least 80 at.%, at least 90 at. %, or at least 95 at. % of the particulate material.In some embodiments, the total amount of Ti present in the particulatematerial is up to 96 at. %, up to 97 at. %, up to 98 at. %, or more ofthe particulate material. Combinations of these ranges are alsopossible. Other values are also possible.

The second metal can be, for example, any of the second metals describedabove.

In some embodiments, at least a portion of the particulates include thesecond metal in an amount of less than or equal to 40 at. %, less thanor equal to 35 at. %, less than or equal to 32 at. %, less than or equalto 30 at. %, less than or equal to 25 at. %, less than or equal to 22at. %, less than or equal to 20 at. %, less than or equal to 15 at. %,or less than or equal to 12 at. %. In some embodiments, at least aportion of the particulates include the second metal in an amount of atleast 1 at. %, at least 2 at. %, at least 3 at. %, at least 4 at. %, atleast 5 at. %, at least 6 at. %, at least 7 at. %, at least 8 at. %, atleast 9 at. %, at least 10 at. %, or more. Combinations of these rangesare also possible. For example, in some embodiments, at least a portionof the particulates include the second metal in an amount of from 1 at.% to 40 at. % of the particulate material. In some embodiments, at leasta portion of the particulates include the second metal in an amount offrom 8 at. % to 32 at. % of the particulate material. Other values arealso possible.

In some embodiments, the total amount of the second metal in theparticulate material is less than or equal to 40 at. %, less than orequal to 35 at. %, less than or equal to 32 at. %, less than or equal to30 at. %, less than or equal to 25 at. %, less than or equal to 22 at.%, less than or equal to 20 at. %, less than or equal to 15 at. %, orless than or equal to 12 at. % of the particulate material. In someembodiments, the total amount of the second metal in the particulatematerial is at least 1 at. %, at least 2 at. %, at least 3 at. %, atleast 4 at. %, at least 5 at. %, at least 6 at. %, at least 7 at. %, atleast 8 at. %, at least 9 at. %, at least 10 at. %, or more of theparticulate material. Combinations of these ranges are also possible.For example, in some embodiments, the total amount of the second metalpresent in the particulate material is from 1 at. % to 40 at. % of theparticulate material. In some embodiments, the total amount of thesecond metal present in the particulate material is from 8 at. % to 32at. % of the particulate material. Other values are also possible.

According to certain embodiments, at least some of the nanocrystallineparticulates are formed by mechanically working a powder comprising theTi and the second metal. For example, certain embodiments comprisemaking nanocrystalline particulates, at least in part, by mechanicallyworking a powder including a plurality of Ti particulates and aplurality of second metal particulates. Certain embodiments comprisemaking nanocrystalline particulates, at least in part, by mechanicallyworking particulates that include both Ti and the second metal.

In embodiments that make use of mechanical working, any appropriatemethod of mechanical working may be employed to mechanically work apowder and form nanocrystalline particulates. According to certainembodiments, at least some of the nanocrystalline particulates areformed by ball milling a powder comprising the Ti and the second metal.The ball milling process may be, for example, a high energy ball millingprocess. In a non-limiting exemplary ball milling process, a tungstencarbide or steel milling vial may be employed, with a ball-to-powderratio of 2:1 to 5:1, and a stearic acid process control agent content of0.01 wt % to 3 wt %. In some embodiments, the mechanical working may becarried out in the presence of a stearic acid process control agentcontent of 1 wt %, 2 wt %, or 3 wt %. According to certain otherembodiments, the mechanical working is carried out in the absence of aprocess control agent. Other types of mechanical working may also beemployed, including but not limited to, shaker milling and planetarymilling. In some embodiments, the mechanical working (e.g., via ballmilling or another process) may be performed under conditions sufficientto produce a nanocrystalline particulate comprising a supersaturatedphase. Supersaturated phases are described in more detail below.

In certain embodiments, the mechanical working (e.g., ball milling) maybe conducted for a time of greater than or equal to 2 hours (e.g.,greater than or equal to 4 hours, greater than or equal to 6 hours,greater than or equal to 8 hours, greater than or equal to 10 hours,greater than or equal to 12 hours, greater than or equal to 15 hours,greater than or equal to 20 hours, greater than or equal to 25 hours,greater than or equal to 30 hours, or greater than or equal to 35hours). In some embodiments, the mechanical working (e.g., ball milling)may be conducted for a time of 1 hour to 35 hours (e.g., 2 hours to 30hours, 4 hours to 25 hours, 6 hours to 20 hours, 8 hours to 15 hours, or10 hours to 12 hours). In some cases, if the mechanical working time istoo long, the Ti and/or the second metal may be contaminated by thematerial used to perform the mechanical working (e.g., milling vialmaterial). The amount of the second metal that is dissolved in the Timay, in some cases, increase with increasing mechanical working (e.g.,milling) time. In some embodiments, after the mechanical working step(e.g., ball milling step), a phase rich in the second metal material maybe present.

According to certain embodiments, the Ti and the second metal arepresent in the particulates in a non-equilibrium phase. The particulatesmay, according to certain embodiments, include a non-equilibrium phasein which the second metal is dissolved in the Ti. In some embodiments,the non-equilibrium phase comprises a solid solution.

According to some embodiments, the non-equilibrium phase may be asupersaturated phase comprising the second metal dissolved in the Ti. A“supersaturated phase,” as used herein, refers to a phase in which amaterial is dissolved in another material in an amount that exceeds thesolubility limit. The supersaturated phase can include, in someembodiments, an activator element and/or a stabilizer element forciblydissolved in the Ti in an amount that exceeds the amount of theactivator element and/or the stabilizer element that could be otherwisedissolved in an equilibrium phase of the Ti. For example, in one set ofembodiments, the supersaturated phase is a phase that includes anactivator element forcibly dissolved in Ti in an amount that exceeds theamount of activator element that could be otherwise dissolved in anequilibrium Ti phase.

In some embodiments, the supersaturated phase may be the only phasepresent after the mechanical working (e.g., ball milling) process. Incertain embodiments, a second phase rich in the second metal may bepresent after the mechanical working (e.g., ball milling) process. Forexample, in some cases, a second phase rich in the activator element maybe present after mechanical working (e.g., ball milling).

According to certain embodiments, the non-equilibrium phase may undergodecomposition during the sintering of the nanocrystalline particulates(which sintering is described in more detail below). The sintering ofthe nanocrystalline particulates may cause the formation of a phase richin the second metal at at least one of the surface and grain boundariesof the nanocrystalline particulates. In some such embodiments, the Ti issoluble in the phase rich in the second metal. The formation of thephase rich in the second metal may be the result of the decomposition ofthe non-equilibrium phase during the sintering. The phase rich in thesecond metal may, according to certain embodiments, act as a fastdiffusion path for the Ti, enhancing the sintering kinetics andaccelerating the rate of sintering of the nanocrystalline particulates.According to some embodiments, the decomposition of the non-equilibriumphase during the sintering of the nanocrystalline particulatesaccelerates the rate of sintering of the nanocrystalline particulates.

Certain, although not necessarily all, embodiments comprise coldpressing the plurality of nanocrystalline particulates during at leastone portion of time prior to the sintering. It has been found that,according to certain embodiments, metal alloys comprising Ti and asecond metal (e.g., Ti and Mg) can be compressed such that high relativedensities are achieved without the need for simultaneous heating. Insome embodiments, the cold pressing comprises compressing of theplurality of nanocrystalline particulates at a force greater than orequal to 300 MPa, greater than or equal to 400 MPa, greater than orequal to 500 MPa, greater than or equal to 750 MPa, greater than orequal to 1000 MPa, greater than or equal to 1500 MPa, greater than orequal to 2000 MPa, or higher. In some embodiments, the cold compressioncomprises compressing the plurality of nanocrystalline particulates at aforce of up to 2500 MPa, or greater. Combinations of these ranges arealso possible (e.g., greater than or equal to 300 MPa and less than orequal to 2500 MPa). Other ranges are also possible.

According to certain embodiments, the cold compression is performed at arelatively low temperature. For example, in some embodiments, the coldcompression is performed while the particulates are at a temperature ofless than or equal to 150° C., less than or equal to 100° C., less thanor equal to 75° C., less than or equal to 50° C., less than or equal to40° C., less than or equal to 35° C., less than or equal to 30° C., lessthan or equal to 25° C., or less than or equal to 20° C. In someembodiments, the cold compression is performed at a temperature of thesurrounding, ambient environment.

As noted above, certain embodiments comprise sintering a plurality ofnanocrystalline particulates to form the nanocrystalline metal alloy.Those of ordinary skill in the art are familiar with the process ofsintering, which involves applying heat to the material (e.g.,particulates) that is to be sintered such that the material becomes asingle solid mass.

FIGS. 1A-1C are exemplary schematic diagrams showing a sinteringprocess, according to certain embodiments. In FIG. 1A, a plurality ofparticulates 100 are shown in the form of spheres (although, asmentioned elsewhere, other shapes could be used). As shown in FIG. 1B,particulates 100 can be arranged such that they contact each other. Asshown in FIG. 1C, as the particulates are heated, they agglomerate toform a single solid material 110. During the sintering process,according to certain embodiments, interstices 105 between particulates100 (shown in FIG. 1B) can be greatly reduced or eliminated, such that asolid having a high relative density is formed (shown in FIG. 1C).

According to certain embodiments, the sintering can be performed whenthe metal particulates are at a relatively low temperature and/or for arelatively short period of time, while maintaining the ability to formmetal alloys having high relative densities, small grain sizes, and/orequiaxed grains.

According to certain embodiments, sintering the plurality ofnanocrystalline particulates involves heating the nanocrystallineparticulates to a sintering temperature of less than or equal to 1200°C., less than or equal to 1100° C., less than or equal to 1000° C., lessthan or equal to 900° C., less than or equal to 850° C., less than orequal to 800° C., less than or equal to 750° C., less than or equal to700° C., less than or equal to 650° C., less than or equal to 600° C.,less than or equal to 550° C., less than or equal to 500° C., less thanor equal to 450° C., less than or equal to 400° C., or less than orequal to 400° C. According to certain embodiments, sintering theplurality of nanocrystalline particulates involves heating thenanocrystalline particulates to a sintering temperature of greater thanor equal to 300° C., greater than or equal to 350° C., greater than orequal to 400° C., greater than or equal to 500° C., greater than orequal to 600° C., greater than or equal to 700° C., or greater than orequal to 900° C. Combinations of these ranges are also possible. Forexample, in some embodiments, sintering the plurality of nanocrystallineparticulates involves heating the nanocrystalline particulates to asintering temperature that is greater than or equal to 300° C. and lessthan or equal to 850° C. In some embodiments, sintering the plurality ofnanocrystalline particulates involves heating the nanocrystallineparticulates to a sintering temperature that is greater than or equal to300° C. and less than or equal to 450° C. In some embodiments, thetemperature of the sintered material is within these ranges for at least10%, at least 25%, at least 50%, at least 75%, at least 90%, or at least99% of the sintering time.

According to certain embodiments, sintering the plurality ofnanocrystalline particulates involves maintaining the nanocrystallineparticulates within the range of sintering temperatures for less than 72hours, less than 48 hours, less than or equal to 24 hours, less than orequal to 12 hours, less than or equal to 6 hours, less than or equal to4 hours, less than or equal to 3 hours, less than or equal to 2 hours,or less than or equal to 1 hour (and/or, in some embodiments, for atleast 10 minutes, at least 20 minutes, at least 30 minutes, or at least50 minutes). Combinations of these ranges are also possible. Forexample, in some embodiments, sintering the plurality of nanocrystallineparticulates involves heating the nanocrystalline particulates to afirst sintering temperature that is greater than or equal to 300° C. andless than or equal to 850° C. for a sintering duration greater than orequal to 10 minutes and less than or equal to 24 hours. In someembodiments, the sintering comprises heating the nanocrystallineparticulates to a temperature greater than or equal to 300° C. and lessthan or equal to 850° C. for a duration greater than or equal to 20minutes and less than or equal to 3 hours. In some embodiments, thesintering comprises heating the nanocrystalline particulates to atemperature greater than or equal to 300° C. and less than or equal to450° C. for a duration greater than or equal to 50 minutes and less thanor equal to 2 hours. In certain embodiments, the sintering comprisesheating the nanocrystalline particulates to a temperature greater thanor equal to 300° C. and less than or equal to 850° C. for a durationgreater than or equal to 10 minutes and less than or equal to 2 hours.

According to certain embodiments, during the sintering step, thenanocrystalline particulates are at highly elevated temperatures foronly a short period of time (or not at all). In some embodiments, thesintering is performed such that the nanocrystalline particulates arenot at a temperature of greater than or equal to 1200° C. (or greaterthan or equal to 1100° C., greater than or equal to 1000° C., greaterthan or equal to 900° C., greater than or equal to 800° C., greater thanor equal to 700° C., greater than or equal to 600° C., greater than orequal to 500° C., greater than or equal to 400° C., or greater than orequal to 300° C.) for more than 24 hours, more than 12 hours, more than6 hours, more than 2 hours, more than 1 hour, more than 30 minutes, morethan 10 minutes, more than 1 minute, more than 10 seconds, or less. Insome embodiments, the sintering is performed such that thenanocrystalline particulates do not exceed a temperature of 1200° C.(or, do not exceed a temperature of 1100° C., do not exceed atemperature of 1000° C., do not exceed a temperature of 900° C., do notexceed a temperature of 800° C., do not exceed a temperature of 700° C.,do not exceed a temperature of 600° C., or do not exceed a temperatureof 500° C.).

According to certain embodiments, sintering comprises heating thenanocrystalline particulates to a first sintering temperature that islower than a second sintering temperature needed for sintering Ti in theabsence of the second metal. To determine whether such conditions weremet, one of ordinary skill in the art would compare the temperaturenecessary to achieve sintering in the sample containing the Ti and thesecond metal to the temperature necessary to achieve sintering in asample containing the Ti without the second metal, but otherwiseidentical to the sample containing the Ti and the second metal. In someembodiments, the first sintering temperature can be at least 25° C., atleast 50° C., at least 100° C., or at least 200° C. lower than thesecond sintering temperature.

According to certain embodiments, a non-equilibrium phase present in thenanocrystalline particulates (e.g., any of the non-equilibrium phasesdescribed above or elsewhere herein) undergoes decomposition during thesintering. In some such embodiments, the decomposition of thenon-equilibrium phase accelerates a rate of sintering of thenanocrystalline particulates.

In some embodiments, the sintering further comprises forming a secondphase at at least one of a surface and a grain boundary of thenanocrystalline particulates during the sintering. In some suchembodiments, Ti is insoluble in the second phase. In some suchembodiments, the second phase is rich in the second metal. The term“rich” with respect to the content of an element in a phase refers to acontent of the element in the phase of at least 50 at. % (e.g., at least60 at. %, at least 70 at. %, at least 80 at. %, at least 90 at. %, atleast 99.%, or higher). The term “phase” is generally used herein torefer to a state of matter. For example, the phase can refer to a phaseshown on a phase diagram.

According to certain embodiments, during the sintering, Ti has a firstdiffusivity in itself and a second diffusivity in a second phase rich inthe second metal, the first diffusivity being larger (e.g., at least 1%,at least 5%, at least 10%, at least 25%, at least 50%, or at least 100%larger) than the second diffusivity.

The sintering may be conducted in a variety of suitable environments. Incertain embodiments, the nanocrystalline particulates are in an inertatmosphere during the sintering process. The use of an inert atmospherecan be useful, for example, when reactive metals are employed in thenanocrystalline particulates. For example, Ti and Mg are reactive witheach other in the presence of oxygen.

In some embodiments, the sintering is performed in an atmosphere inwhich at least 90 vol. %, at least 95 vol. %, at least 99 vol. %, orsubstantially all of the atmosphere is made up of an inert gas. Theinert gas can be or comprise, for example, helium, argon, xenon, neon,krypton, combinations of two or more of these, or other inert gas(es).

In certain embodiments, oxygen scavengers (e.g., getters) may beincluded in the sintering environment. The use of oxygen scavengers canreduce the degree to which the metals are oxidized during the sinteringprocess, which may be advantageous according to certain embodiments. Insome embodiments, the sintering environment can be controlled such thatoxygen is present in an amount of less than 1 vol. %, less than 0.1 vol.%, less than 100 parts per million (ppm), less than 10 ppm, or less than1 ppm.

According to certain embodiments, the sintering is conducted essentiallyfree of external applied stress. For example, in some embodiments, forat least 20%, at least 50%, at least 75%, at least 90%, or at least 98%of the time during which sintering is performed, the maximum externalpressure applied to the nanocrystalline particulates is less than orequal to 2 MPa, less than or equal to 1 MPa, less than or equal to 0.5MPa, or less than or equal to 0.1 MPa. The maximum external pressureapplied to the nanocrystalline particulates refers to the maximumpressure applied as a result of the application of a force external tothe nanocrystalline particulates, and excludes the pressure caused bygravity and arising between the nanocrystalline particulates and thesurface on which the nanocrystalline particulates are positioned duringthe sintering process. Certain of the sintering processes describedherein can allow for the production of relatively highly dense sinteredultra-fine and nanocrystalline materials even in the absence orsubstantial absence of external pressure applied during the sinteringprocess. According to certain embodiments, the sintering may be apressureless sintering process.

According to certain embodiments, at least one activator element may bepresent during the sintering process. The activator element may enhancethe sintering kinetics of Ti. According to certain embodiments, theactivator element may provide a high diffusion path for the Ti atoms.For example, in some embodiments, the activator element atoms maysurround the Ti atoms and provide a relatively high transport diffusionpath for the Ti atoms, thereby reducing the activation energy ofdiffusion of the Ti. In some embodiments, this technique is referred toas activated sintering. The activator element may, in some embodiments,lower the temperature required to sinter the nanocrystallineparticulates, relative to the temperature that would be required tosinter the nanocrystalline particulates in the absence of the activatorelement but under otherwise identical conditions. Thus, the sinteringmay involve, according to certain embodiments, a first sinteringtemperature, and the first sintering temperature may be lower than asecond sintering temperature needed for sintering the Ti in the absenceof the second metal. To determine the sintering temperature needed forsintering the Ti in the absence of the second metal, one would prepare asample of the Ti material that does not contain the second metal but isotherwise identical to the nanocrystalline particulate material. Onewould then determine the minimum temperature needed to sinter the samplethat does not include the second metal. In some embodiments, thepresence of the second metal lowers the sintering temperature by atleast 25° C., at least 50° C., at least 100° C., at least 200° C., ormore.

According to certain embodiments, at least one stabilizer element may bepresent during the sintering process. The stabilizer element may be anyelement capable of reducing the amount of grain growth that occurs,relative to the amount that would occur in the absence of the stabilizerelement but under otherwise identical conditions. In some embodiments,the stabilizer element reduces grain growth by reducing the grainboundary energy of the sintered material, and/or by reducing the drivingforce for grain growth. The stabilizer element may, according to certainembodiments, exhibit a positive heat of mixing with the sinteredmaterial. The stabilizer element may stabilize nanocrystalline Ti bysegregation in the grain boundaries. This segregation may reduce thegrain boundary energy, and/or may reduce the driving force against graingrowth in the alloy.

In some embodiments, the stabilizer element may also be the activatorelement. The use of a single element both as the stabilizer andactivator elements has the added benefit, according to certainembodiments, of removing the need to consider the interaction betweenthe activator and the stabilizer. In some embodiments, the element thatmay be utilized as both the activator and stabilizer element may be ametal element, which may be any of the aforedescribed metal elements.

According to certain embodiments, when one element cannot act as boththe stabilizer and the activator, two elements may be employed. Theinteraction between the two elements may be accounted for, according tosome embodiments, to ensure that the activator and stabilizer roles areproperly fulfilled. For example, when the activator and the stabilizerform an intermetallic compound each of the elements may be preventedfrom fulfilling their designated role, in some cases. As a result,activator and stabilizer combinations with the ability to formintermetallic compounds at the expected sintering temperatures should beavoided, at least in some instances. The potential for the formation ofintermetallic compounds between two elements may be analyzed with phasediagrams.

According to one set of embodiments, titanium powders and magnesiumpowders (e.g., 10, 20, or 30 at. % Mg with the balance being titanium)can be mechanically alloyed via ball milling, cold compressed, andsubsequently annealed (e.g., in a thermomechanical analyzer for severalhours). In some embodiments, the Ti—Mg alloy system exhibitsnanocrystalline grain size stabilization by formation of a nano-duplexstructure.

According to certain embodiments, powders of elemental Ti and Mg aremixed and milled to achieve supersaturation and a decrease of the grainsize to the nanometer scale. In some embodiments, annealing ofcompressed powders leads to the development of a nano-duplex structureconsisting of Ti-rich grains and Mg-rich precipitates. In someembodiments, a nanocrystalline structure with grain sizes of around 110nm can be maintained even after 8 hours at 500° C. (which is 84% of themelting temperature for Mg and 30% for Ti). In some embodiments, highrelative densities can be achieved for Ti-20 at. % Mg and Ti-30 at. %Mg. It is believed that this may indicate that accelerated densificationis possible.

U.S. Provisional Application No. 62/384,232, filed Sep. 7, 2016, andentitled “Stable Nano-Duplex Titanium-Magnesium Alloys” is incorporatedherein by reference in its entirety for all purposes.

The following example is intended to illustrate certain embodiments ofthe present invention, but does not exemplify the full scope of theinvention.

Example

This example demonstrates how processing by low-temperature, acceleratedsintering methods were able to be applied to produce nanocrystallinetitanium-magnesium (Ti—Mg) alloys with thermal stability and highrelative density.

Titanium powders with different additions of magnesium powders (10, 20,and 30 at. % Mg) were mechanically alloyed via high-energy ball millingin a stainless steel vial and stainless steel media. With this process,supersaturated powders with microcrystalline particles andnanocrystalline grain sizes were produced after milling times of around15 hours. The powders were then cold compressed and subsequentlysintered in pure argon atmosphere. The microstructure of the milledpowders consisted of supersaturated titanium grains with sizes of around10 to 20 nm. After sintering (also referred to herein as “annealing”) to600° C., the grain size increased to around 100 nm and separated intotitanium-rich and magnesium-rich grains. Even after prolonged sinteringtimes, the structure remained stable.

Accelerated sintering (pressureless) of nanocrystalline alloys wasconducted. Production of supersaturated powders was accomplished viahigh-energy ball milling. It is believed that the sintering involvedprecipitation and neck formation of solute on solvent. The effect ofnecks may have involved fast solute diffusion due to excess vacancies,and diffusion of solvent within necks due to solubility of solvent insolute, that resulted in enhanced densification.

FIG. 2 shows the enthalpy of segregation, ΔH^(seg) (kJ/mol), and theenthalpy of mixing, ΔH^(mix) (kJ/mol) of various metals with titanium.Magnesium (an alkaline earth metal) was determined to be a goodcandidate to alloy with titanium along with scandium (Sc) and yttrium(Y) (transition metals), thorium (Th) (an actinide), lanthanum (La) (alanthanide), chromium (Cr), silver (Ag), iron (Fe), manganese (Mn),copper (Cu), and lithium (Li). This is because a positive enthalpy ofmixing led to phase separation and a positive enthalpy of segregationled to grain boundary segregation, which stabilized the nanocrystallinestructure. As shown in FIG. 2 , Mg is in the nano-duplex region with Tifor nano-phase separation in a solvent-rich and solute-rich phase. ATi—Mg phase diagram showed a large miscibility gap (not shown). Themelting point of Mg is 650° C., much less than the melting temperatureof Ti at 1668° C.

FIG. 3 shows a series of x-ray diffraction (XRD) spectra fornanocrystalline powder samples that contained titanium and 20 at. %magnesium (Ti-20 at. % Mg) that were processed by high-energy ballmilling at 1000 cycles per minute for 0 hours, 2 hours, 4 hours, 8hours, 12 hours, 16 hours, and 20 hours, using 5 grams of Ti—Mg mixtureplus 1 wt. % stearic acid. Ti peaks moved to lower angles and Mg peaksdisappeared, which demonstrated the supersaturation of Mg in Ti of thepowders during milling. There was dissolution of magnesium in titanium.XRD patterns after 0 to 20 hours milling demonstrated supersaturation ofTi-20 at. % Mg powder during milling and a decrease of the grain size(peak shift to lower angles and peak broadening). FIG. 4 shows adistinct decrease of grain size below 20 nm after 16 hours milling andincrease of the lattice parameters c and a for all mixtures. Inaddition, FIG. 4 demonstrates that a supersaturated phase was formed.

Processing of Ti-xMg was done with x=10 at. %, 20 at. %, and 30 at. %.High-energy ball milling of mixed elemental powders Ti-xMg (x=10 at. %,20 at. %, 30 at. %) was conducted to produce supersaturated powders.

FIG. 4 shows a plot of grain sizes of nanocrystalline powders thatcontained titanium and 10 at. % Mg, 20 at. % Mg, and 30 at. % Mgmeasured from x-ray diffraction (XRD) and transmission electronmicroscopy (TEM) that were made by high-energy ball milling at 1000cycles per minute for 0 hours, 2 hours, 4 hours, 8 hours, 12 hours, 16hours, and 20 hours, using 5 grams of Ti—Mg mixture plus 1 wt. % stearicacid. Milling recipe: using steel vial and media, ball-to-powder ratio10:1, 1 weight percent (wt. %) stearic acid, and milling time: e.g. 20hours. As can be seen in FIG. 4 , grain size decreased drastically asmilling time increased.

FIG. 5 shows a series of TEM images and corresponding electrondiffraction patterns for nanocrystalline powders that contained titaniumand 10 at. % Mg, 20 at. % Mg, and 30 at. % Mg that were made byhigh-energy ball milling at 1000 cycles per minute for 20 hours, using 5grams of Ti—Mg mixture plus 1 wt. % stearic acid. As indicated in FIG. 4, average grain sizes for these powders were 18 nm for 10 at. % Mg, 15nm for 20 at. % Mg, and 10 nm for 30 at. % Mg, as measured by imageanalysis. The scale bar in all TEM images in FIG. 5 is 30 nm. Thecontinuous rings in the electron diffraction patterns are characteristicof nanocrystalline samples in general.

FIG. 6 shows an electron diffraction pattern from TEM of Ti-20 at. % Mgafter high-energy ball milling as described above, and diffraction ringsof supersaturated titanium with Miller-Bravais indices (10-10), (0002),(10-11), (10-12), and (10-20) (corresponding to a hexagonal close-packedcrystal structure) have been superimposed on the pattern for emphasis.Table 1 shows: d is the distance between atomic planes; d_(calculated)was calculated with the lattice parameter using the Bragg equation; andd_(measured) was measured in the diffraction pattern.

TABLE 1 Diffraction rings of supersaturated titanium. Miller-BravaisIndex d_(calculated) d_(measured) (10-10) 0.221 0.221 (0002) 0.185 0.205(10-11) 0.171 0.195 (10-12) 0.101 0.148 (10-20) 0.073 0.128

Compacted samples were sintered in a thermomechanical analyzer(Netzsch), using standard parameters for the instrument. Sintering ofcold compressed samples (h=4 mm, d=6 mm) was carried out in thethermomechanical analyzer, isothermally held at e.g. 500° C. for 8 hoursto determine the stability of the nanocrystalline structure, and with aconstant heating rate of e.g. 5 K/min to e.g. 550° C. to explore thesintering behavior. First, cold compression of powders was carried outwith different loads (1 t-6 t), where the conversion from t to MPa istabulated in Table 2.

TABLE 2 Conversion from t to MPa. Applied Load Resulting Pressure [t][MPa] 1 347 2 694 3 1041 4 1388 5 1735 6 2058

A representative sample size had a height of approximately 4 mm, and adiameter of approximately 6 mm. Samples were covered with tantalum (Ta)foil (FIG. 7A) or copper (Cu) tube (FIG. 7B). Sintering under isothermalconditions and with constant heating rate was carried out. Theisothermal condition was at from 400° C. to 600° C. (e.g., 500° C.) for8 h. The constant heating rate condition was a heating rate of from 5K/min to 20 K/min (e.g., 5 K/min) to a maximum temperature of from 550°C. to 700° C. FIG. 8 demonstrates the effect of the applied load duringcold compression (in t) on the relative density (%) of thenanocrystalline alloy Ti with 20 at. % Mg. Green bodies were onlypressed at room temperature, and sintered samples were sintered using aramp rate of 5 K/min to 600° C. The powders had first been high-energyball milled at 1000 cycles per minute for 20 hours, using 5 grams ofTi—Mg mixture plus 1 wt. % stearic acid. The relative density wasmeasured using dimensions of the sample and then calculated using thetheoretical density of the sample. As FIG. 8 demonstrates, compaction togreater than 80% relative density was achieved in green bodies andgreater than 95% relative density in sintered samples.

Table 3 shows the melting temperature of titanium and magnesium alone(T_(m)), half of the melting temperature of Ti and Mg where thehalf-melt temperature was calculated by first converting to Kelvin(0.5·Tm), and room temperature relative to the melting temperature oftitanium and magnesium where the calculation was made by firstconverting to Kelvin (RT). Table 3 shows that compared to ordinarysintering, the sintering temperatures used in this example are very low.

TABLE 3 Melting temperatures and related temperatures for titanium andmagnesium. 0.5 · T_(m) [° C.]; RT (25° C.); T_(m) determined determined[° C.] using Kelvin using Kelvin Ti 1668 698 0.15 · T_(m) Mg 650 1890.32 · T_(m)

FIG. 9 shows the in situ progression of the relative density (%) forsamples sintered to from 550° C. to 600° C. for different compositionsof Ti—Mg alloys. The final relative density of the alloy depended atleast in part on composition, compaction pressure during coldcompression of the Ti—Mg powder, and sintering temperature. Thedeviation of the curves can be attributed to the Ta foil in which thesamples were encased being rigid.

The microstructure after sintering for the Ti—Mg alloys was analyzed byscanning transmission electron microscopy-energy dispersive x-rayspectroscopy (STEM-EDS). FIGS. 10A-10C show STEM-EDS of Ti-20 at % Mgalloy after having sintered at 500° C. for 8 h (scale bar 600 nm). TheEDS map of Ti (FIG. 10B) shows that the titanium was concentratedprimarily in the light gray continuous region of the STEM image (FIG.10A). The EDS map of Mg (FIG. 10C) shows that the magnesium wasconcentrated primarily in the black isolated regions of the STEM image(FIG. 10A).

FIG. 11 shows an XRD pattern comparison before (dotted) and after (solidlines) sintering. The Ti peaks shifted back in the direction of pure Tiand narrowed. Small Mg peaks occurred after sintering, which agreed withthe occurrence of a Mg-rich phase and some grain growth depicted in theSTEM results.

FIGS. 12, 13, and 10A show STEM images of different Ti—Mg alloycompositions after sintering at 500° C. for 8 hours in pure Aratmosphere and STEM-EDS images of the Ti and Mg distribution for Ti-20at. % Mg. FIG. 12 (Ti-10 at. % Mg, d=119 nm, scale bar=300 nm), FIG. 13(Ti-30 at. % Mg, d=126 nm, scale bar=300 nm), and FIG. 10A (Ti-20 at. %Mg, d=107 nm, scale bar=300 nm), are the STEM images, and FIGS. 10B and10C are the STEM-EDS images for Ti-20 at. % Mg. The grain size wasstabilized at on average 110 nm and the grain structure of all threesamples showed a well-developed nano-duplex structure comprising Ti-richgrains and Mg-rich precipitates, shown by the element distribution inthe STEM-EDS images.

Table 4 shows the grain size after sintering for Ti-10 at. % Mg, Ti-20at. % Mg, and Ti-30 at. % Mg alloys. In addition, Table 4 indicates thechange in relative density between the cold compressed powder and theresulting sintered alloy. Grain sizes were determined by TEM and XRD.FIG. 9 shows the change of the relative density during sintering with aconstant heating rate of 5 K/min to 550° C. for different Ti—Mg alloys.A distinct densification above 350° C. occurred for Ti-20 at. % Mg andTi-30 at. % Mg. Cold compression led to higher relative densities thanexpected. Relative densities of greater than 90% after sintering wereachieved.

TABLE 4 Relative Density and Grain Size Data for Ti—Mg Alloys. HoldingRelative Relative Grain Cold Heating Final Time at density after densitySize after Compression Rate Temp. Temp. sintering Change Sintering (MPa)(K/min) (° C.) (hours) (%) [%] [nm] Ti-10Mg 1735 40 500 8 78.5 −1.9 119Ti-10Mg 347 40 500 8 63.2 −0.6 144 Ti-10Mg 347 5 550 0 60.6 −0.6 87Ti-20Mg 347 40 400 8 87.6 4.5 53 Ti-20Mg 1735 40 500 8 89.0 5.6 107Ti-20Mg 347 40 500 8 67.6 7.6 149 Ti-20Mg 694 40 500 24 78.4 13.1 185Ti-20Mg 347 40 600 6 73.8 12.1 270 Ti-20Mg 347 40 600 24 76.3 14.5 385Ti-20Mg 1735 5 550 0 91.9 8.0 109 Ti-20Mg 347 5 550 0 60.8 2.6 92Ti-20Mg 1735 5 550 0 90.0 7.2 Ti-20Mg 1735 5 600 0 91.7 8.6 Ti-20Mg 20825 600 0 89.6 7.7 Ti-20Mg 1735 5 700 0 93.9 12.9 Ti-20Mg 347 5 700 0 78.614.0 242 Ti-20Mg 347 10 800 0 79.9 16.4 Ti-30Mg 1735 40 500 8 93.5 5.8126 Ti-30Mg 347 40 500 8 59.9 8.9 144 Ti-30Mg 694 5 550 0 63.4 7.8 105Ti-30Mg 2082 5 600 0 96.1 7.0 157 Ti-30Mg 347 10 550 0 69.4 8.0 Ti-30Mg347 5 600 0 74.4 14.1 119 Ti-30Mg 347 10 600 0 77.4 13.2 113 Ti-30Mg 34715 600 0 75.5 10.6 90 Ti-30Mg 347 20 600 0 75.0 12.4 95

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used. Those skilled in the art will recognize,or be able to ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described and claimed. Thepresent invention is directed to each individual feature, system,article, material, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,and/or methods, if such features, systems, articles, materials, and/ormethods are not mutually inconsistent, is included within the scope ofthe present invention.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Other elements may optionallybe present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

1-72. (canceled)
 73. A method of forming a nanocrystalline metal alloy,comprising: sintering a plurality of nanocrystalline particulates toform the nanocrystalline metal alloy; wherein at least some of thenanocrystalline particulates comprise Ti and a second metal, and Ti isthe most abundant metal by atomic percentage in at least some of thenanocrystalline particulates; and wherein for at least 20% of the timeduring which sintering is performed, the maximum external pressureapplied to the nanocrystalline particulates is less than or equal to 2MPa.
 74. The method of claim 73, wherein the nanocrystalline metal alloyhas a relative density of at least 80%.
 75. The method of claim 73,wherein the nanocrystalline metal alloy has a relative density of atleast 98%.
 76. The method of claim 75, wherein the nanocrystalline metalalloy is a bulk nanocrystalline metal alloy.
 77. The method of claim 76,wherein the bulk nanocrystalline metal alloy has an average grain sizeof less than 300 nm.
 78. The method of claim 77, wherein the secondmetal is selected from the group consisting of Mg, La, Y, Th, Sc, Cr,Ag, Fe, Mn, Cu, and Li.
 79. The method of claim 78, wherein the secondmetal is Mg.
 80. The method of claim 73, wherein the Ti and the secondmetal are present in a non-equilibrium phase.
 81. The method of claim80, wherein the non-equilibrium phase comprises a solid solution. 82.The method of claim 80, wherein the non-equilibrium phase undergoesdecomposition during the sintering.
 83. The method of claim 82, whereinthe decomposition of the non-equilibrium phase accelerates a rate ofsintering of the nanocrystalline particulates.
 84. The method of claim80, wherein the non-equilibrium phase comprises a supersaturated phasecomprising the second metal dissolved in Ti.
 85. The method of claim 73,further comprising forming at least some of the nanocrystallineparticulates by mechanically working a powder comprising Ti and thesecond metal.
 86. The method of claim 73, wherein the second metal isselected from the group consisting of Mg, La, Y, Th, Sc, Cr, Ag, Fe, Mn,Cu, and Li.
 87. The method of claim 73, wherein the second metal is Mg.88. The method of claim 73, wherein sintering the plurality ofnanocrystalline particulates involves heating the nanocrystallineparticulates such that the nanocrystalline particulates are not at atemperature of greater than or equal to 1200° C. for more than 24 hours.89. The method of claim 73, further comprising cold pressing theplurality of nanocrystalline particulates during at least one portion oftime prior to the sintering.
 90. The method of claim 89, wherein thecold pressing comprises cold compression of the plurality ofnanocrystalline particulates at a force greater than or equal to 300 MPaand less than or equal to 2500 MPa.
 91. The method of claim 73, whereinthe sintering comprises heating the nanocrystalline particulates to afirst sintering temperature lower than a second sintering temperatureneeded for sintering Ti in the absence of the second metal.
 92. Themethod of claim 73, wherein the sintering comprises heating thenanocrystalline particulates to a temperature greater than or equal to300° C. and less than or equal to 850° C. for a duration greater than orequal to 10 minutes and less than or equal to 24 hours.